Japanese Patent Application Laid-Open No. 2005-295248, which was laid open to public inspection on Oct. 20, 2005, discloses a high frequency oscillation type proximity sensor, that is configured such that an object constituted by metal (conductor) or magnetic substance is sensed without contact. In concrete terms, this proximity sensor includes a parallel resonant circuit having a sensing coil and a capacitor. If this object approaches the parallel resonance circuit, an eddy circuit loss is generated in the object due to an electromagnetic induction function, which changes the impedance (effective resistance value) of the sensing coil, and also changes the oscillation conditions of the parallel resonant circuit. Because of these changes, the proximity sensor senses the presence of this object when the oscillation of the oscillation signal, that is supplied to the parallel resonant circuit, stops or when the amplitude of the oscillation signal is attenuated by a specified value or more.
This proximity sensor, however, only generates a binary signal that shows whether the object exists in a sensing range corresponding to this specified value, and cannot provide a distance signal corresponding to the distance between the object and the sensing coil. Therefore this proximity sensor cannot show how close the object approached the sensing coil.